1. Field of the Invention
The present invention is directed to an adaptive delta modulator of the type having an analog-to-digital converter (AND-converter) controlled by a digital control signal.
2. Description of the Prior Art
Basically, delta modulation is a simple method of converting analog signals into digital signals. A basic delta modulator (see FIG. 1A ) consists of a comparator and a sampler in a direct path and an integrator-amplifier in a feedback path. The analog input signal m(t) is compared with the feedback signal {circumflex over (m)}(t). If there is an error signal xcex5(t)={circumflex over (m)}(t)xe2x88x92m(t) this is applied to a comparator. If xcex5 is positive, the comparator output is a constant signal of amplitude E, and if xcex5 is negative, the comparator output is xe2x88x92E. Thus the comparator output m,(t) is given by mc(t)=E sign xcex5(t). The comparator output is sampled by a sampler at a rate of fs samples/second where fs is typically much higher than the Nyquist rate. The sampler thus produces a pulse train d(t) which consists of positive pulses when {circumflex over (m)}(t) greater than m(t) and negative pulses when {circumflex over (m)}(t) less than m(t). The pulse train d(t) is the delta modulated pulse train. The modulated signal d(t) is amplified and integrated in the feedback path to generate {circumflex over (m)}(t) (FIG. 1B), which is to follow m(t). Each pulse in d(t) (FIG. 1C) gives rise to a step function (positive or negative depending on the pulse polarity) in {circumflex over (m)}(t) . The use of only one step size in the above described delta modulator limits the possibilities to follow input signals having rapid changes. If, however, the input signal has small changes a relatively large step size would make the corresponding digital output signal too erratic. A solution to this problem is accomplished by increasing or decreasing the step sizes used by the delta modulator. This well known device is called an adaptive delta modulator.
In U.S. Pat. No. 4,527,133 a delta modulator is described. One problem that the device of U.S. Pat. No. 4,527,133 is directed to solve is the problem emanating from that the two current sources are not perfectly matched. That means that one of the current sources will have to to be connected to the input capacitor more often than the other in order to control the same total voltage change across the capacitor albeit in opposite directions). According to U.S. Pat. No. 4,527,133 this problem is solved by arranging an additional capacitor to which periodically both current sources are connected simultaneously. The voltage across this capacitor is a function of the mismatch in the two current source amplitudes. The voltage across the capacitor is used to control the amplitude of at least one of the two current sources. Thus, the drift compensation is made by arranging extra hardware. It is an overall ambition in the field of implantable medical devices, such as implantable heart stimulators or defibrillators, to minimize the volume and weight and to minimize the energy consumption of the components used in such devices. For that reason the solution of the problem of drift compensation of the current sources used in U.S. Pat. No. 4,527,133 suffers from the drawback of comprising extra hardware rendering the device to voluminous and heavy and energy consuming and thus not suitable for use in modern implantable devices. A light and less energy consuming delta modulator according to the invention can, of course, be used in any technical field. In practice one way of realizing a delta modulator is principally by arranging a capacitor that is charged by one current source and discharged by another current source so that the potential of the capacitor tracks the analog input signal. In the case of an adaptive delta modulator a predetermined number of current generators are arranged for charging and discharging wherein each current generator generates a predetermined unique current.
It is an object of the present invention to provide an adaptive delta modulator wherein the above-described drift compensation problem is solved.
This object is achieved in accordance with the principles of the present invention in an adaptive delta modulator having an A/D-converter controlled by a digital control signal, the digital control signal also being supplied to a drift compensation logic circuit which generates a drift compensated digital output signal by manipulation of the control signal, in order to produce an output signal of the delta modulator.